This invention relates generally to an accumulator that holds a pressurized hydraulic working fluid and automatically replenishes a volume of gas that keeps the accumulator charged.
Fuel economy of a vehicle can be increased by a stop-start system that automatically stops an internal combustion engine when the vehicle is stopped and restarts the engine when an operator indicates intent to accelerate the vehicle. A vehicle equipped with an automatic transmission and the stop-start system requires that automatic transmission fluid be maintained pressurized while the engine is stopped. An engine driven hydraulic pump that normally pressurizes the automatic transmission fluid while the engine is running is incapable of maintaining the fluid pressurized while the engine is stopped. An accumulator containing pressurized transmission fluid is continually connected to the hydraulic system of the transmission so that hydraulic system pressure is maintained until the engine restarts. The accumulator uses a gas pre-charge to maintain the pressure of the transmission fluid.
However, slow leakage through and around a piston seal in the accumulator depletes the gas pre-charge over the life of a gas-charged accumulator. This changes the amount of working fluid that the accumulator holds and eventually lowers the peak working pressure of the accumulator when the piston reaches a stroke limit.